A polarization element means an optical element having a function of transmitting a linearly polarized wave having an electric field vibration plane in a specific direction and preventing the transmission of a linearly polarized wave having an electric field vibration plane in a direction perpendicular to the specific direction (hereinafter, in the polarization element, the electric field vibration direction of transmission light is referred to as a “transmission axis” and a direction perpendicular to the transmission axis is referred to as an “extinction axis”).
The characteristics of the polarization element are evaluated using loss (hereinafter, the loss is referred to as an “insertion loss”) when a linearly polarized wave having an electric field vibration plane in a direction parallel to the transmission axis of the element passes through the polarization element and a value (hereinafter, the value is referred to as an “extinction ratio”) obtained by dividing the intensity of the electric field when a linearly polarized wave having an electric field vibration plane parallel to the extinction axis passes through the polarization element by the intensity of the electric field when a linearly polarized wave having an electric field vibration plane parallel to the transmission axis passes through the polarization element. The polarization element with good characteristics means an element with a small insertion loss and a high extinction ratio.
As the polarization element using metal fine particles, a polarizing glass has been known in which acicular metal fine particles made of silver or copper are dispersed in a glass substrate such that the longitudinal direction thereof is aligned with a specific direction (hereinafter, the polarization element is referred to as a “metal fine particle dispersed polarizing glass”). A method of manufacturing the metal fine particle dispersed polarizing glass is described in detail in, for example, JP-A-5-208844 and the main points thereof are as follows.
<1> A glass material including cuprous chloride is prepared to have a desired composition, is melted at a temperature of about 1450° C., and is then annealed to the room temperature. <2> Then, a thermal treatment is performed to precipitate cuprous chloride fine particles in the glass. <3> After the cuprous chloride fine particles are precipitated, a preform with an appropriate shape is produced by a machining process. <4> The preform is heated and elongated under predetermined conditions, thereby obtaining acicular particles of cuprous chloride. <5> The elongated cuprous chloride is reduced in a hydrogen atmosphere to obtain acicular metallic copper fine particles.
A metal fine particle dispersed polarizing glass in which acicular metallic copper fine particles with a minor axis of 20 to 75 nm, a major axis of 120 to 350 nm, and an aspect ratio (value obtained by dividing the major axis of the acicular particle by the minor axis thereof) of 2.5 to 10 are dispersed in a base glass is manufactured by the manufacturing method, which is disclosed in JP-A-5-208844.
Patent Citation 1: JP-A-5-208844